Contents

DeltaRPM Support (Presto)

Summary

The presto plugin for yum adds support for downloading deltarpms and using them to generate new packages. If we ship and enable this plugin by default, we can make a substantial dent in the amount of data having to be downloaded by our users for updates.

Usage cases/rationale

Justin only has a dial-up connection available for his computer. Currently he has very little chance to actually keep up with the update churn of Fedora due to the amount of data being transferred. Adding presto support will reduce the amount of data he has to transfer and thus make it so that he can keep his Fedora system updated.

Barry pays for his network connection based on usage. Adding presto support will reduce the amount of data he has to transfer and thus reduce the cost for his network connection.

Scope

Requires that the yum-presto plugin be included by default. Also requires some infrastructure work on the buildsystem and compose tools so that we can ensure that deltas are included in the -updates repositories.

Test Plan

Ensure that updates work using deltas rather than downloading full packages.

Ensure that changing files from a package lead to downloading the full package rather than the delta.

Ensure that changing a %config file from a package does not lead to downloading the full package.

Ensure that yum doesn't break when yum-presto runs into problems. (i.e. always fall back to downloading the full package if nothing else works)

Dependencies

The binary diff algorithm used in deltarpm is a version of bsdiff modified for speed.

Details

Contingency Plan

Documentation

Release Notes

Normally when you update a package in Fedora, you download an entire replacement package. Most of the time (especially for the larger packages), most of the actual data in the updated package is the same as the original package, but you still end up downloading the full package. Presto allows you to download the difference (called the delta) between the package you have installed and the one you want to update to. This can reduce the download size of updates by 60% - 80%.